Field of the Invention
The present invention generally relates to fiber optic transducers and more particularly, to a transducer utilizing a single optical fiber for detecting displacement of a deformable reflective surface with respect to the optical fiber, wherein the optical fiber is cut at an angle to result in frustrated total internal reflection.
Fiber optic transducers are used in many applications. For example, fiber optic transducers are particularly suited for sensing conditions, such as temperature and pressure in a hazardous environment containing explosive and flammable vapors and liquids, since such transducers do not generate electrical arcs as conventional electrical switches. Also, due to the relatively light weight of the optical fibers they are also suitable for weight constrained applications, such as aboard aircraft requiring relatively long runs to remote switches. Fiber optic transducers are also used for interfacing with optical logic devices.
Various fiber optic transducers are known in the art. Such transducers are generally used to sense conditions, such as pressure and temperature and provide an optical signal at a remote location which can be converted to an electrical signal and displayed. Both analog and digital fiber optic transducers are known.
Some known fiber optic transducers utilize two or more optical fibers. For example, Iwamoto et al U.S. Pat. No. 4,687,927 discloses a fiber optic pressure transducer. In this device, light is transmitted from a light source via a first optical fiber to a pressure responsive diaphragm having a reflective surface which reflects light as a function of the pressure applied to the diaphragm. The reflected light is applied to a second optical fiber which transmits the reflected light to a photosensitive element. In order to optimize the reflected light from the diaphragm, the end surfaces of the optical fibers are terminated with respect to the reflecting surface of the diaphragm at an oblique angle.
In Snider U.S. Pat. No. 4,588,886, a fiber optic transducer is disclosed for measuring temperature and pressure. This transducer utilizes a bundle of optical fibers, terminated such that the axes of the fibers are generally perpendicular to the reflected surface on the diaphragm.
The problem with such transducers utilizing two or more fibers is that such devices are relatively complex and require precision alignments at the reflective surfaces and the adjoining fibers. Such devices are also relatively more expensive and more complex; inherently reducing the reliability of the system.
Optical transducers utilizing single optical fibers are also known. For example, Perlin U.S. Pat. No. 4,678,902 discloses a fiber optic transducer utilizing a single optical fiber. In this transducer, the geometry of the fiber is altered in such a manner to allow light exiting the fiber to project onto a reflected surface in an expanded cone. In one embodiment this is achieved by bending the optical fiber adjacent its exit end. In another embodiment, the axis of the optical fiber is terminated so as to be relatively parallel to the plane of the reflective surface. The end of the optical fiber is then cut at an angle to cause light reaching the end surface to be reflected back into the optical fiber, rather than being allowed to exit the end surface. The reflective light then exits the circumference of the optical fiber. Such a fiber optic transducer requires relatively close control of the geometry of the optical fiber with respect to the sensing element.
Anderson et al U.S. Pat. No. 4,703,174 also discloses a fiber optic transducer, which utilizes a single optical fiber. Both a pressure sensitive transducer and a temperature sensitive transducer are disclosed. The pressure sensitive transducer includes a pressure sensitive piston, disposed in a cylindrical cavity within the sensor housing. A cylindrical carrier element is disposed within the sensor housing which moves coaxially with respect to the piston. A reflective surface is disposed on one end of the carrier adjacent the optical fiber.
The Anderson et al patent also discloses a temperature sensitive fiber optic transducer. In this embodiment, a pair of bimetallic strips are longitudinally disposed within a cylindrical housing. The bimetallic strips are attached at the top to a cylindrical carrier. The other ends of the bimetallic strips are anchored to the bottom of the transducer housing. As the temperature sensor is exposed to heat the bimetallic strips bow inwardly. This causes the cylindrical carrier to move axially within the cylindrical housing.
In both embodiments, a reflective surface is disposed on one end of the carrier adjacent the fiber optic cable. As the carrier moves toward and away from the optical fiber, the amount of light reflected back into the optical fiber from the reflective surface varies. This change in reflectance is used to determine the temperature or pressure.
A transducer, such as disclosed in the Anderson et al patent, is relatively complicated and requires precise alignment of the components over time, even though the transducer may be exposed to shock and vibration. Also, initial alignment may involve relatively expensive procedures and require the use of relatively close tolerance components.
Other fiber optic transducers are known which use polarization techniques. However, these transducers require polarization retaining fibers which are generally more costly and require control of the angular alignment during the assembly process. Other known trandsducers utilize nonpolarization retaining fibers which are sensitive to bends and stresses which affect the polarization. Moreover, either of the two fibers must be aligned or reflective element used to modulate the output signal using polarization.
In other known fiber optic transducers, which utilize snap acting bimetallic discs, the optical fibers are often attached directly to the disc. This creates several problems associated with stresses caused by bending, which can affect the stability of the attachment joint which, in turn, affects the switch point accuracy. Also there are relatively high dynamic forces involved when switching such a snap acting disc which can affect both the radial and axial alignment of the optical fiber.